The present invention relates to a coupling structure for an antenna device that transmits or receives various types of radio waves.
An electronic key system installed in a vehicle uses an electronic key as a vehicle key that transmits a unique key code through wireless communication to the vehicle. One type of such an electronic key system is a wireless key system that requires the operation of a button to transmit the key code. In such a wireless key system, when a lock button of the electronic key is pushed, a lock request radio wave, which includes a key code, is transmitted from the electronic key. Upon receipt of the lock request radio wave, the vehicle locks the unlocked doors if the key code in the radio wave is correct. When an unlock button of the electronic key is pushed, an unlock request radio wave, which includes the key code, is transmitted from the electronic key. When the key code in the radio wave is correct, the vehicle unlocks the locked doors.
The electronic key system includes an antenna, which is installed in the vehicle to receive various types of radio waves transmitted from the electronic key. One example of such an antenna is an inverted L antenna. The inverted L antenna has the shape of inverted letter L from the alphabet. Japanese Laid-Open Patent Publication No. 2003-8331 describes an example of an inverted L antenna. FIG. 1 is a schematic diagram showing the structure described in the publication. As shown in the drawing, an antenna device 105 includes an inverted L antenna 110. The inverted L antenna 110 includes a generally U-shaped antenna element 112, which has a vertical end extending orthogonal to a substrate 107 and a horizontally extending portion bent twice by 90 degrees. The antenna element 112 is arranged on a conductive surface, which is larger than the antenna element 112, and has a length set to be, for example, one fourth the wavelength. In this case, the vehicle body or substrate that is larger than the wavelength functions as the conductive surface. As the size of the conductive surface becomes greater than the wavelength, the antenna properties are further stabilized.
A wire harness 130 is connected to the substrate 107, on which the antenna 100 is mounted, to connect the antenna device 105 to another device. However, when coupling the antenna device 105 to a vehicle body or the like, the layout situation (e.g., length and position) of the wire harness 130 differs in accordance with the application. For example, the length of the antenna element 112 is determined by the wavelength. However, when the antenna device 105 is required to be reduced in size, the substrate 107 must also be miniaturized. Further, in the inverted L antenna 110, the antenna element 112 does not function as an antenna by itself. Rather, the antenna element 112 affects the substrate 107 to function in the same manner as a dipole. Thus, the image produced on a ground plane of the substrate 107 affects the antenna properties. Moreover, the conductive surface may have an area that is not sufficiently larger than the wavelength. In such a case, when the layout situation of the wire harness 130 differs depending on the application, the wire harness 130, which is a conductor, functions as the ground plane and may thereby vary the antenna directivity. This may destabilize the antenna properties. Accordingly, it is required that the antenna properties be stabilized in an antenna device that is connected to a wire harness.